Process and apparatus for producing single-walled carbon nanotubes

ABSTRACT

A process and apparatus for catalytic production of single walled carbon nanotubes. Catalytic particles are exposed to different process conditions at successive stages wherein the catalytic particles do not come in contact with reactive (catalytic) gases until preferred process conditions have been attained, thereby controlling the quantity and form of carbon nanotubes produced. The reaction gas is preferably provided at a high space velocity to minimize CO 2  build-up. The process also contemplates processes and apparatus which recycle and reuse the gases and catalytic particulate materials, thereby maximizing cost efficiency, reducing wastes, reducing the need for additional raw materials, and producing the carbon nanotubes, especially SWNTs, in greater quantities and for lower costs.

RELATED REFERENCES

This is a continuation of U.S. Ser. No. 09/996,142, filed Nov. 28, 2001,which is a continuation-in-part of U.S. Ser. No. 09/587,257, filed Jun.2, 2000. U.S. Ser. No. 09/996,142 also claims the benefit of the filingdate of U.S. Provisional Application 60/253,877, filed Nov. 29, 2000.The specification of each of the above is hereby expressly incorporatedherein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was supported by NSF Grant CTS-9726465, the U.S.Government has certain rights herein.

BACKGROUND OF THE INVENTION

This invention is related to the field of producing carbon nanotubes,and more particularly, but not by way of limitation, to processes andapparatus for producing single-walled carbon nanotubes. Thespecification of each of U.S. Ser. No. 09/587,257, U.S. ProvisionalApplication 60/253,877, and U.S. Ser. No. 09/389,593 have subject matterwhich is relevant to the present invention and each is hereby expresslyincorporated herein by reference in its entirety.

Carbon nanotubes (also referred to as carbon fibrils) are seamless tubesof graphite sheets with full fullerene caps which were first discoveredas multilayer concentric tubes or multi-walled carbon nanotubes andsubsequently as single-walled carbon nanotubes in the presence oftransition metal catalysts. Carbon nanotubes have shown promisingapplications including nanoscale electronic devices, high strengthmaterials, electron field emission, tips for scanning probe microscopy,and gas storage.

Generally, single-walled carbon nanotubes are preferred overmulti-walled carbon nanotubes for use in these applications because theyhave fewer defects and are therefore stronger and more conductive thanmulti-walled carbon nanotubes of similar diameter. Defects are lesslikely to occur in single-walled carbon nanotubes than in multi-walledcarbon nanotubes because multi-walled carbon nanotubes can surviveoccasional defects by forming bridges between unsaturated carbonvalances, while single-walled carbon nanotubes have no neighboring wallsto compensate for defects.

However, the availability of these new single-walled carbon nanotubes inquantities necessary for practical technology is still problematic.Large scale processes for the production of high quality single-walledcarbon nanotubes are still needed.

Presently, there are three main approaches for synthesis of carbonnanotubes. These include the laser ablation of carbon (Thess, A. et al.,Science, 273:483, 1996), the electric arc discharge of graphite rod(Journet, C. et al., Nature, 388:756, 1997), and the chemical vapordeposition of hydrocarbons (Ivanov, V. et al., Chem. Phys. Lett,223:329, 1994; Li A. et al., Science, 274:1701, 1996). The production ofmulti-walled carbon nanotubes by catalytic hydrocarbon cracking is nowon a commercial scale (U.S. Pat. No. 5,578,543) while the production ofsingle-walled carbon nanotubes is still in a gram scale by laser(Rinzler, A. G. et al., Appl. Phys. A., 67:29, 1998) and arc (Journet,C. et al., Nature, 388:756, 1997) techniques.

Unlike the laser and arc techniques, carbon vapor deposition overtransition metal catalysts tends to create multi-walled carbon nanotubesas a primary product instead of single-walled carbon nanotubes. However,there has been some success in producing single-walled carbon nanotubesfrom the catalytic hydrocarbon cracking process. Dai et al. (Dai, H. etal., Chem. Phys. Lett, 260:471 1996) demonstrate web-like single-walledcarbon nanotubes resulting from disproportionation of carbon monoxide(CO) with a molybdenum (Mo) catalyst supported on alumina heated to1200° C. From the reported electron microscope images, the Mo metalobviously attaches to nanotubes at their tips. The reported diameter ofsingle-walled carbon nanotubes generally varies from 1 nm to 5 nm andseems to be controlled by the Mo particle size. Catalysts containingiron, cobalt or nickel have been used at temperatures between 850° C. to1200° C. to form multi-walled carbon nanotubes (U.S. Pat. No.4,663,230). Recently, rope-like bundles of single-walled carbonnanotubes were generated from the thermal cracking of benzene with ironcatalyst and sulfur additive at temperatures between 1100-1200° C.(Cheng, H. M. et al., Appl. Phys. Lett., 72:3282, 1998; Cheng, H. M. etal., Chem. Phys. Lett., 289:602, 1998). The synthesized single-walledcarbon nanotubes are roughly aligned in bundles and woven togethersimilarly to those obtained from laser vaporization or electric arcmethod. The use of laser targets comprising one or more Group VI orGroup VIII transition metals to form single-walled carbon nanotubes hasbeen proposed (WO98/39250). The use of metal catalysts comprising ironand at least one element chosen from Group V (V, Nb and Ta), VI (Cr, Moand W), VII (Mn, Tc and Re) or the lanthanides has also been proposed(U.S. Pat. No. 5,707,916). However, methods using these catalysts havenot been shown to produce quantities of nanotubes having a high ratio ofsingle-walled carbon nanotubes to multi-walled carbon nanotubes.Moreover, metal catalysts are an expensive component of the productionprocess.

In addition, the separation steps which precede or follow the reactionstep represent a large portion of the capital and operating costsrequired for the production of the carbon nanotubes. Therefore, thepurification of single-walled carbon nanotubes from multi-walled carbonnanotubes and contaminants (i.e., amorphous and graphitic carbon) may besubstantially more time consuming and expensive than the actualproduction of the carbon nanotubes.

Therefore, new and improved methods of producing nanotubes which enablesynthesis of bulk quantities of substantially pure single-walled carbonnanotubes at reduced costs are sought. It is to such methods andapparatus for producing nanotubes that the present invention isdirected.

SUMMARY OF THE INVENTION

According to the present invention, a process and apparatus forproducing carbon nanotubes is provided which avoids the defects anddisadvantages of the prior art. Broadly, the process includescontacting, in a reactor cell, metallic catalytic particles with aneffective amount of a carbon-containing gas at a temperature sufficientto catalytically produce carbon nanotubes, wherein a substantial portionof the carbon nanotubes are single-walled.

Further, the invention contemplates a process wherein the catalyticparticles are exposed to different process conditions at successivestages, wherein the catalytic particles do not come in contact withreactive (catalytic) gases until preferred process conditions have beenattained, thereby controlling the quantity and form of carbon nanotubesproduced. The process also contemplates methods and apparatus whichrecycle and reuse the gases and catalytic particulate materials, therebymaximizing cost efficiency, reducing wastes, reducing the need foradditional raw materials, and producing the carbon nanotubes, especiallysingle walled carbon nanotubes, in greater quantities and for lowercosts.

Other objects, features and advantages of the present invention willbecome apparent from the following detailed description when read inconjunction with the accompanying figures and appended claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a flowchart showing the process steps of one embodiment of thepresent invention.

FIG. 2 is a cross-sectional view of a reactor which can be used with theprocess contemplated as one embodiment of the present invention.

FIG. 3 is a cross-sectional view through line 3-3 of the reactor of FIG.2.

FIG. 4 is a diagrammatic representation of an apparatus which can beused in the process of the present invention.

FIG. 5 is a diagrammatic representation of another apparatus which canbe used in the process of the present invention.

FIG. 6 is a graphic representation of calculated equilibrium conversionsof the Boudouard Reaction, in a continuous reactor operating at constantpressure and with pure CO in the feed, as a function of the totalpressure for different temperatures in the range of 600° C. to 1200° C.

FIG. 7 is a graphic representation of calculated equilibrium conversionsof the Boudouard Reaction, in a continuous reactor operating at constantpressure and at a temperature of 800° C., as a function of the totalpressure for different inlet concentrations of CO in the range of10%-100% CO in He.

FIG. 8 is a graphic representation of carbon yield as a function ofreaction time for different reaction conditions, using the catalystCo:Mo(1:3)/SiO₂ as a fine powder at 700° C. under the followingconditions: (●) reaction carried out in reactor A with 100 cm³/min of50% CO in He; (▴) reaction carried out in reactor B with 50 cm³/min of50% CO in He; (♦) reaction carried out in reactor B with 100 cm³/min of50% CO in He; (Δ) reaction carried out in reactor B with 50 cm³/min of100% CO; (⋄) reaction carried out in reactor B with 100 cm³/min of 100%CO; (□) reaction carried out in reactor B with 135 cm³/min of 100% CO.

FIG. 9 is a graphic representation of carbon yield as a function ofreaction time using reactor B with the catalyst Co:Mo(1:3)/SiO₂ as afine powder and operating at 700° C. with 50 sccm of 100% CO and apressure in the inlet of the reactor of: (Δ) about 15 psi or (∘) about75 psi.

FIG. 10 is a graphic representation of carbon yield along reactor C thatis measured by the mass of catalyst from the inlet to the outlet whenthe reaction is carried out for 120 minutes under a pressure of 85 psiand the following conditions: (♦) 850 sccm of 100% CO, 800° C.; (⋄) 850sccm of 100% CO, 800° C., injecting CO₂ for 90 seconds just beforestarting the reaction; (Δ) 850 sccm of 100% CO, 700° C.; (□) 850 sccm of50% CO in He, 800° C.; (▪) 850 sccm of 50% CO in CO₂, 800° C.

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of a process contemplated by the inventiondescribed herein is characterized by the schematic flowchart shown inFIG. 1. The process shown in FIG. 1 is but one embodiment of the presentinvention and as such it is to be understood that the present inventionis not limited to this example or to other examples shown herein.

FIG. 1 shows a series of process steps A-Q which represent a process ofcontinuous catalytic production of carbon nanotubes. In Step A, aquantity of catalytic particles is introduced into a reactor, such asbut not limited to, the reactor 10 described elsewhere herein in detailand shown in FIGS. 2 and 3. The catalytic particles can be any particlecomprising a catalyst effective in forming carbon nanotubes. Especiallypreferred embodiments of the catalytic particles are described elsewhereherein, but it will be understood that the present invention is not tobe limited only to the types of catalytic particles explicitly describedherein. In any event, the catalytic particles generally comprise a solidsupport material which first has been impregnated with a metalliccatalyst (i.e., a transition metal precursor), then calcined, thenpreferably processed into a pellet form. The pelletization process canbe performed either before or after the support material is impregnatedwith the catalyst (transition metal precursor).

The present process is especially designed for the production ofsingle-walled carbon nanotubes (SWNTs) because in the present processthe reaction conditions (e.g., temperature and duration of exposure toreaction conditions) to which the catalytic particles are exposed arehighly controlled at different stages. The ability to regulatetemperature and reactive concentrations is important to obtain the highselectivity necessary to produce SWNTs. In the process described herein,these problems have been solved by subdividing the process and thereactor in which the process steps occur into different stages so thatthe catalytic particles are not contacted with the reactive gas (e.g.,CO) until the optimal reaction conditions have been achieved. Forexample, the yield of nanotubes is affected by the catalyst formulation(e.g., transition metal ratio, type of support, and metal loading), bythe operating parameters (e.g., reaction temperature, catalytic gaspressure, space velocity and reaction time), and by pretreatmentconditions (e.g., reduction and calcination).

After the catalytic particles have been introduced into the reactor,Step B is carried out in which the catalytic particles are treated witha heated inert gas, e.g., He, under high pressure, which functions bothto preheat the catalytic particles to a high temperature, such as 700°C. for example, and to remove air from the catalytic particles inpreparation for the subsequent reduction step. In Step C, the catalyticparticles are exposed to a reducing gas such as H₂ at 500° C. under highpressure which reduces, at least partially, the catalyst within thecatalytic particles to prepare it for catalysis, and the reducing gas isflushed from the catalytic particles by an inert gas such as He heatedto 750° C. under high pressure, for example, which also reheats thecatalytic particles for the next step. Where used herein, the term “highpressure” or “elevated pressure” is intended to generally represent apressure in the range of from about 1 atm to about 40 atm, where apressure of about 6 atm is preferred. Other elevated pressure levels maybe used in other versions of the invention contemplated herein.

Step D follows Step C and is the reaction step in which an effectiveamount of a carbon-containing gas such as CO is heated to a suitablereaction temperature, such as 750° C., and is exposed to the reducedcatalytic particles under high pressure. It is during this stage of theprocess that carbon nanotubes and amorphous carbon are formed on thecatalytic particles. Note that before the catalytic particles have beenexposed to the carbon-containing gas, the reducing gas, e.g., H₂, hasbeen flushed from the flow of catalytic particles by the reheating gas,e.g., an inert gas such as He under high pressure.

After Step D, the catalytic particles are subjected to a Step E in whichthe reacted catalytic particles are exposed to a heated post reactiongas under high pressure, such as He heated to 750° C., for example,which functions to flush the carbon-containing gas remaining from theprevious Step D. Then the flushed catalytic particles are cooled with acooling gas such as He or other inert gas under high pressure at a lowertemperature, such as 300° C. or lower. After the reacted catalyticparticles have been cooled, they are subjected to Step F wherein theyare exposed to a stream of a heated oxidative gas such as O₂ at 300° C.under high pressure, for example, wherein the amorphous carbon particlesare burned away from the catalytic particles, substantially leaving onlycarbon nanotubes in the catalytic particles. In Step G, the oxidizedcatalytic particles are then removed from the reactor for furtherprocessing. In Step H, the catalytic particles are subjected to apurification process which results in the separation of the catalyst(which bears the nanotubes) from the support. In a preferred process,the support, such as SiO₂, is dissolved by treatment with a base, suchas NaOH, at a concentration of 0.1-1.0 Molar, at a preferred temperatureof from about 22° C. to about 70° C. and with vigorous stirring,sonication or any other appropriate method known to those of ordinaryskill in the art. Alternatively, the support may be soluble in an acidrather than a base, for example, a MgO support, alumina support, or ZrO₂support. Such supports will be dissolved by treatment with an acid, suchas HCl, HF, HNO₃, aqua regia, or a sulfo-chromic mixture, under similarconditions to that described hereinbefore. Other support materials mayrequire other methods of separation from the catalyst, e.g., usingorganic solvents such as chloro-compounds, and are also considered to beencompassed by the bounds of the present invention. For example, in analternative embodiment organic solvents can be used to separate thecarbon nanotubes from silica support by extraction after sonicationusing methods known in the art.

The term “catalyst” where used herein may also be used interchangeablywith any of the terms “catalyst material,” “metallic catalyst,” “metalcatalyst,” “transition metal” and “transition metal precursor.” The term“support” may be used interchangeably herein with the term “supportmaterial” or “support component.”

After the support has been separated from the catalyst, the catalyst isfurther treated in Step I by exposure to strong acid (e.g., 0.1 M to 9M), thereby causing dissolution of the catalyst and separation of thecatalyst from the nanotubes, thereby yielding a purified form of thecarbon nanotubes in Step J. The carbon nanotubes can then be furtherprocessed to yield carbon nanotubes having a greater purity.

A key aspect of the present invention is to recycle and reuse thesupport material and catalyst material to improve the economy of thenanotube production process. Reuse of the metal catalyst is importantbecause the metal catalyst is one of the most expensive components ofthe entire process. The support is recovered in Step K by precipitationfrom solution obtained during Step H, wherein precipitation of thesupport occurs by neutralization of the solvent (i.e., base or acid).“Fresh” support can be added in Step M to the support precipitated inStep K to make up for support material lost during the process.

Similarly, the metal catalyst is recovered in Step L by precipitationfrom solution when the acid (or other dissolution solution) isneutralized. “Fresh” catalyst can be added in Step N to catalystrecovered in Step L to make up for catalyst material lost during theprevious steps of the process.

The precipitated support and catalyst materials from Steps K and L,including the fresh support and catalyst materials added in Steps M andN, are combined in Step O wherein the support material and catalyst aretreated using methods well known to those of ordinary skill in the artto cause the support material to be impregnated with the catalyst. Theimpregnated support is then calcined and pelletized in Step P, againusing methods well known in the art, to form the catalytic particles tobe fed into the reactor. If desired, in Step Q, additional “fresh”catalytic particles can be added at this stage and combined with thecatalytic particles from Step P, which together are then fed into thereactor, thereby completing the process of the present invention. StepsO and P can be modified in any manner which is effective in regeneratingthe catalytic particles for use in the reactor.

Benefits and advantages of the carbon nanotube production processcontemplated herein are numerous. The process as contemplated herein canbe adjusted to maximize the production of SWNTs due to the fact that theprocess conditions and parameters can be highly controlled. The processis economical because the process is continuous (although it may beprocessed in a “batch”) and because materials and gases used in theprocess are recovered and recycled. Recycling reduces the amount ofwaste product as well as the amount of raw materials initially required,thereby reducing the overall cost of the process. The process results inthe catalytic particles being exposed to each gaseous phase for aminimum duration, thereby maintaining a more constant reactantconcentration (e.g., minimizing CO₂ buildup) which is favorable forobtaining a homogenous nanotube product. The process contemplated hereinfurther enables use of high gas flow rates, thereby minimizing theexternal diffusional effects and maximizing the heat transfer rate. Asnoted earlier, the solid phase (catalytic particles) retention time canbe adjusted independent of the gas phases. This enables the process andapparatus contemplated herein to be used with a wide range of catalystswith different activities. Further, the process is independent of thereaction yield, and the division into separate stages and steps allowsdifferent thermal treatments to be used. These factors enableoptimization of the gas high space velocity. Additionally, as noted,initial purification of the product can be done within the reactor (theoxidation or “combustion” step).

In general, the process for producing single-walled carbon nanotubescomprises contacting catalytic particles with an effective amount of acarbon-containing gas heated to a temperature of from about 500° C. toabout 1200° C., preferably from about 600° C. to about 1000° C., andmore preferably from about 650° C. to about 950° C., and more preferablyfrom about 750° C. to about 850° C.

The phrase “an effective amount of a carbon-containing gas” as usedherein means a gaseous carbon species present in sufficient amounts toresult in deposition of carbon on the catalytic particles at elevatedtemperatures, such as those described herein, resulting in formation ofcarbon nanotubes.

As noted elsewhere herein, the catalytic particles as described hereininclude a catalyst preferably deposited upon a support material. Thecatalyst as provided and employed in the present invention is preferablybimetallic and in an especially preferred version contains at least onemetal from Group VIII, which includes Co, Ni, Ru, Rh, Pd, Ir, Pt, and atleast one metal from Group VIb, which includes Cr, W, and Mo or fromGroup Vb, which includes V, Nb, and Ta. Specific examples of bimetalliccatalysts which may be employed by the present invention include Co—Cr,Co—W, Co—Mo, Co—Nb, Ni—Cr, Ni—W, Ni—Mo, Ru—Cr, Ru—W, Ru—Mo, Rh—Cr, Rh—W,Rh—Mo, Pd—Cr, Pd—W, Pd—Mo, Ir—Cr, Ir—W, Ir—Mo, Pt—Cr, Pt—W, and Pt—Mo.Especially preferred catalysts of the present invention include Co—Mo,Co—W, Ni—Mo and Ni—W. The catalyst may comprise more than one of themetals from each group.

A synergism exists between the two metal components of a bimetalliccatalyst in that metallic catalytic particles containing a bimetalliccatalyst are much more effective catalysts for the production ofsingle-walled carbon nanotubes than metallic catalytic particlescontaining either a Group VIII metal, a Group VIb metal, or a Group Vbmetal alone as the catalyst.

The ratio of the Group VIII metal to the Group VIb or Group Vb metal inthe metallic catalytic particles where a bimetallic catalyst is used mayalso affect the selective production of single-walled carbon nanotubes.The ratio of the Group VIII metal to the Group VIb or Group Vb metal ina bimetallic catalyst is preferably from about 1:10 to about 15:1, andmore preferably about 1:5 to about 2:1. Preferably, the concentration ofthe Group VIb or Group Vb metal (e.g., Mo) will exceed the concentrationof the Group VIII metal (e.g., Co) in metallic catalytic particlesemployed for the selective production of single-walled carbon nanotubes.

The metallic catalytic particles may comprise more than one metal fromeach of Groups VIII and VIb or Vb. For example, the metallic catalyticparticles may comprise (1) more than one Group VIII metal and a singleGroup VIb or Group Vb metal, (2) a single Group VIII metal and more thanone Group VIb or Group Vb metal, or (3) more than one Group VIII metaland more than one Group VIb or Group Vb metal, and in a preferredversion excludes Fe.

The catalyst particles may be prepared by simply impregnating thesupport with the solutions containing the transition metal prescursors.The catalyst can also be formed in situ through decomposition of aprecursor compound such as bis (cyclopentadienyl) cobalt or bis(cyclopentadienyl) molybdenum chloride.

The catalyst is preferably deposited on a support such as silica (SiO₂),MCM-41 (Mobil Crystalline Material-41), alumina (Al₂O₃), MgO, Mg(Al)O(aluminum-stabilized magnesium oxide), ZrO₂, molecular sieve zeolites,or other oxidic supports known in the art.

The metallic catalytic particle, that is, the catalyst deposited on thesupport, may be prepared by evaporating the metal mixtures over flatsubstrates such as quartz, glass, silicon, and oxidized silicon surfacesin a manner well known to persons of ordinary skill in the art.

The total amount of bimetallic catalyst deposited on the support mayvary widely, but is generally in an amount of from about 1% to about 20%of the total weight of the metallic catalytic particle, and morepreferably from about 3% to about 10% by weight of the metalliccatalytic particle.

In an alternative version of the invention, the bimetallic catalyst maynot be deposited on a support, in which case the metal componentscomprise substantially 100% of the metallic catalytic particle.

Examples of suitable carbon-containing gases which may be used hereininclude aliphatic hydrocarbons, both saturated and unsaturated, such asmethane, ethane, propane, butane, hexane, ethylene and propylene; carbonmonoxide; oxygenated hydrocarbons such as acetone, acetylene andmethanol; aromatic hydrocarbons such as toluene, benzene andnaphthalene; and mixtures of the above, for example carbon monoxide andmethane. Use of acetylene promotes formation of multi-walled carbonnanotubes, while CO and methane are preferred feed gases for formationof single-walled carbon nanotubes. The carbon-containing gas mayoptionally be mixed with a diluent gas, such as helium, argon orhydrogen.

In an especially preferred embodiment of the process claimed herein, thecatalytic particle formulation is a Co—Mo/silica support catalyst, witha Co:Mo molar ratio of about 1:2 to about 1:3 to about 1:4. MonometallicCo catalysts or those with a higher Co:Mo ratio tend to result in lowselectivity for SWNTs with significant production of defectivemulti-walled nanotubes and graphite. In the temperature rangeinvestigated, without Co, Mo alone is essentially inactive for nanotubeproduction. The catalytic particles are pretreated with a heated inertgas at a high temperature, such as He at about 700° C., and then treatedwith a reducing gas, such as hydrogen, at about 500° C. Without thepre-reduction step, or with pre-reduction at higher temperatures (i.e.,not enough reduction or too much reduction) the catalyst is noteffective and produces fewer SWNTs. Other supports such as alumina mayresult in a poor Co—Mo interaction, resulting in losses of selectivityand yield.

A high space velocity (above 30,000 h⁻¹) is preferred to minimize theconcentration of CO₂, a by-product of the reaction, which inhibits theconversion to nanotubes. A high CO (or other reactive gas) concentrationis preferred to minimize the formation of amorphous carbon depositswhich occur at low CO (reactive gas) concentrations. The preferredtemperature range is characterized in that below 650° C. the selectivitytoward SWNTs is low; and above 1000° C., the conversion is low due tothe reversibility of the reaction and the deactivation of the catalyst.Therefore, the optimal temperature is between about 700° C. and about1000° C.; more preferably between about 750° C. and about 950° C. andeven more preferably between about 800° C. and 900° C.

The production process contemplated herein has been designed in such away to effect a rapid contact of the preferred catalyst formulation witha flow of highly concentrated CO (or other reactive gas) at about 750°C.-950° C. The quality of the SWNTs produced by this process may bedetermined by a combination of characterization techniques involvingRaman Spectroscopy, Temperature Programmed Oxidation (TPO) and ElectronMicroscopy (TEM).

The preferred methodology therefore comprises contacting a flow of COgas (or other reactive gas in a high concentration) over the catalyticparticles at from about 750° C. to about 950° C. for 1 hour at a highspace velocity (above 30,000/h⁻¹) under high pressure (above 70 psi).

If the conditions indicated above are followed, a high yield of SWNT(about 20-25 grams of SWNTs per 100 grams of initial catalyst loaded inthe reactor) and high selectivity (>90%) is obtained.

Operation

A preferred embodiment of an apparatus for carrying out the processcontemplated herein is shown in FIGS. 2 and 3. The apparatus is areactor identified by reference numeral 10. The reactor 10 isconstructed of three concentric chambers, an inner chamber 12, a middlechamber 14 having an inner space 15 (also referred to herein as a lumen)and an outer chamber 16. The inner chamber 12 is subdivided into aplurality of inlet (gas receiving) chambers including a preheating gasinlet chamber 20 a, a reducing gas inlet chamber 20 b, a reheating gasinlet chamber 20 c, a reaction gas inlet chamber 20 d, a post reactiongas inlet chamber 20 e, a cooling gas inlet chamber 20 f, and acombustion gas inlet chamber 20 g. Each gas inlet chamber 20 a-20 g hasat least one corresponding gas inlet, 22 a-22 g, respectively, and hasat least one corresponding gas outlet 24 a-24 g, respectively. The innerchamber 12 further comprises a closed upper end 26 and a closed lowerend 28.

The middle chamber 14 has an upper end 30 (also referred to herein as aninput end) which has an input conduit 32 for feeding catalytic particlesinto the middle chamber 14, and has a lower end 34 (also referred toherein as an output end) which has an output conduit 36 for removingreacted catalytic particles from the reactor 10. The middle chamber 14is constructed at least partially of a porous material (including, forexample, a perforated metal or screen) for forming a porous (orperforated) wall portion 38 of the middle chamber 14. The porousmaterial may be any material which is permeable to gas introduced intothe reactor 10 but which is impermeable to catalytic particlesintroduced into the inner space 15 contained by the middle chamber 14and which can withstand the operating conditions of the reactor 10. Suchmaterials are known to persons of ordinary skill in the art. The entirereactor 10 must be constructed of materials able to withstand theprocess conditions to which they are exposed, as will be understood by aperson of ordinary skill in the art.

The outer chamber 16 is constructed of a plurality of outlet chambersincluding a preheating gas outlet chamber 40 a, a reducing gas outletchamber 40 b, a reheating gas outlet chamber 40 c, a reaction gas outletchamber 40 d, a post reaction gas outlet chamber 40 e, a cooling gasoutlet chamber 40 f, and a combustion gas outlet chamber 40 g. Each gasoutlet chamber 40 a-40 g has a porous wall portion 42 a-42 g,respectively, for receiving gas into each gas outlet chamber 40 a-40 g,and has at least one corresponding gas outlet 44 a-44 g, respectively,through which gas is eliminated from each corresponding outlet chamber40 a-40 g, respectively.

Each gas outlet chamber 40 a-40 g is positioned across from each gasinlet chamber 20 a-20 g such that gas leaving each gas inlet chamber 20a-20 g under high pressure passes across the porous wall portions 42a-42 g, respectively, and into each gas outlet chamber 40 a-40 g,respectively.

In use, a quantity of catalytic particles 48 are continuously fed intothe reactor 10 through the input conduit 32, and into the inner space 15of the middle chamber 14. An inert preheating gas 50 a is introducedunder high pressure through gas inlet 22 a into the preheating gas inletchamber 20 a and therefrom through gas outlet 24 a, whereby the inertpreheating gas 50 a heats the catalytic particles 48 which are adjacentto the preheating gas inlet chamber 20 a to a desired predeterminedtemperature. The inert preheating gas 50 a then passes across the porouswall portion 42 a into the preheating gas outlet chamber 40 a and out ofthe preheating gas outlet chamber 40 a via gas outlet 44 a. In apreferred embodiment, the preheating temperature is about 700° C.-1000°C., but in alternative embodiments the preheating temperature can be inthe range of from about 500° C. to about 1200° C.

After the catalytic particles 48 have been heated they are moved into aposition adjacent to the reducing gas inlet chamber 20 b and are reducedby a heated reducing gas 50 b, such as H₂, which is introduced underhigh pressure through gas inlet 22 b into the reducing gas inlet chamber20 b and therefrom through gas outlet 24 b wherein the heated reducinggas 50 b passes across the catalytic particles 48 at a high spacevelocity (at least 30,000 h⁻¹), through the porous wall portion 42 b,into the reducing gas outlet chamber 40 b, and out of the reducing gasoutlet chamber 40 b via gas outlet 44 b. In a preferred embodiment, thetemperature of the heated reducing gas 50 b is about 500° C., but inalternative embodiments the temperature of the heated reducing gas 50 bmay be in the range of from about 400° C. to about 800° C. The heatedreducing gas 50 b may be H₂, NH₃, CH₄, or mixtures of NH₃, CH₄, H₂and/or other gases, as long as the heated reducing gas 50 b functions inaccordance with the present invention. In a preferred embodiment, H₂ isutilized as the heated reducing gas 50 b.

After the catalytic particles 48 have been reduced by the heatedreducing gas 50 b, they are moved into a position adjacent to thereheating gas inlet chamber 20 c and are reheated after being cooledduring reduction by an inert reheating gas 50 c, such as He, which isintroduced under high pressure through gas inlet 22 c into the reheatinggas inlet chamber 20 c and therefrom through gas outlet 24 c, whereinthe reheating gas 50 c passes across catalytic particles 48 at a highspace velocity (at least 30,000 h⁻¹), through the porous wall portion 42c, into the reheating gas outlet chamber 40 c, and out of the reheatinggas outlet chamber 40 c via gas outlet 44 c. In a preferred embodimentthe temperature of the reheating gas 50 c is about 750° C.-950° C., butin alternative embodiments the temperature of the reheating gas 50 c isin the range of from about 600° C. to about 1200° C. The reheating gas50 c may be He, Ar, N₂ or other inert gases or mixtures thereof.Preferably, the reheating gas 50 c is He.

After the catalytic particles 48 have been reheated by reheating gas 50c, they are moved into a position adjacent to the reaction gas inletchamber 20 d and are exposed to a heated carbon-containing reaction gas50 d, such as CO, which is introduced under high pressure through gasinlet 22 d into the reaction gas inlet chamber 20 d and therefromthrough gas outlet 24 d, wherein the heated carbon-containing reactiongas 50 d passes across catalytic particles 48, through the porous wallportion 42 d, into the reaction gas outlet chamber 40 d, and out of thereaction gas outlet chamber 40 d via gas outlet 44 d. This stage of theprocess is shown in detail in FIG. 3. In a preferred embodiment, thetemperature of the heated carbon-containing reaction gas 50 d is about750° C., but in alternative embodiments the temperature of the heatedcarbon-containing reaction gas 50 d is in the range of from about 500°C. to about 1200° C. The heated carbon-containing reaction gas 50 d maybe CO, CH₄, C₂H₄, or C₂H₂ or mixtures thereof, or any carbon-containinggas which functions in accordance with the present invention. In apreferred embodiment, CO is utilized as the carbon-containing gas 50 d.

After the catalytic particles 48 have been reacted with the heatedcarbon-containing reaction gas 50 d, they are moved into a positionadjacent to the post reaction gas inlet chamber 20 e and are flushed ofthe heated carbon-containing reaction gas 50 d while at the reactiontemperature by a heated post reaction gas 50 e, such as He, which isintroduced under high pressure through gas inlet 22 e into the postreaction gas inlet chamber 20 e and therefrom through gas outlet 24 e,wherein the heated post reaction gas 50 e passes across catalyticparticles 48, through the porous wall portion 42 e, into the postreaction gas outlet chamber 40 e, and out of the post reaction gasoutlet chamber 40 e via the gas outlet 44 e. In a preferred embodiment,the temperature of the heated post reaction gas 50 e is about 750° C.,i.e., the same temperature as the heated reaction gas 50 d, but inalternative embodiments the temperature of the heated post reaction gas50 e is in the range of from about 300° C. to about 800° C. The postreaction gas 50 e may be He, N₂, Ar, or any other inert gas or mixturesthereof which function in accordance with the present invention. In apreferred embodiment, the post reaction gas 50 e is He.

After the catalytic particles 48 have been cleared of the heatedcarbon-containing reaction gas 50 d by the heated post reaction gas 50e, they are moved into a position adjacent to the cooling gas inletchamber 20 f and are cooled in preparation for combustion of amorphouscarbon by the cooling gas 50 f, such as He, which is introduced underhigh pressure through gas inlet 22 f into the cooling gas inlet chamber20 f and therefrom through gas outlet 24 f, wherein the cooling gas 50 fpasses across catalytic particles 48, through the porous wall portion 42f, into the cooling gas outlet chamber 40 f, and out of the cooling gasoutlet chamber 40 f via the gas outlet 44 f. In a preferred embodiment,the temperature of the cooling gas 50 f is considerably lower than thetemperature of the post reaction gas 50 e, for example about 22° C., butin alternative embodiments the temperature of the cooling gas 50 f is inthe range of from about 0° C. to about 300° C. Ideally, the temperatureof the cooling gas 50 f is a moderate temperature sufficient to cool thecatalytic particles 48 to a temperature lower than or about equal tothat under which the following step will be carried out. The cooling gas50 f may be He, N₂, Ar, or other inert gases or mixtures thereof. In apreferred embodiment, the cooling gas 50 f is He.

After the catalytic particles 48 have been cooled by cooling gas 50 f,they are moved into a position adjacent combustion gas inlet chamber 20g wherein the amorphous carbon residue produced during the reaction stepcan be burned off in a combustion (oxidation) step (without affectingthe nanotubes) by a heated combustion gas 50 g containing O₂ (at aconcentration range of from about 2% to about 5%) which is introducedunder high pressure through gas inlet 22 g into the combustion gas inletchamber 20 g and therefrom through gas outlet 24 g, wherein the heatedcombustion gas 50 g passes across catalytic particles 48, through theporous wall portion 42 g, into the combustion gas outlet chamber 40 g,and out of the combustion gas outlet chamber 40 g via the gas outlet 44g. In a preferred embodiment, the temperature of the heated combustiongas 50 g is about 300° C., but in alternative preferred embodiments thetemperature of the heated combustion gas 50 g is in the range of fromabout 280° C. to about 320° C. The heated combustion gas 50 g may be O₂in a gas mixture, air or an air mixture with He or may be any other gas,as long as the heated combustion gas 50 g functions in accordance withthe present invention to cause oxidation of the amorphous carbon on thecatalytic particles 48. Preferably, the heated combustion gas 50 g is2-5% O₂ in a gas mixture.

After the catalytic particles 48 have been subjected to the oxidationprocess to remove amorphous carbon, they are moved to the lower end 34of the middle chamber 14 of the reactor 10 and are passed out of thereactor 10 through the output conduit 36 for further purification andprocessing as explained elsewhere herein.

While the apparatus for inputting, driving, and outputting the catalyticparticles 48 into, through, and out of the reactor 10 are not shown,such mechanisms are well known to one of ordinary skill in the art, andmay include devices such as slide valves, rotary valves, table feeders,screw feeders, screw conveyors, cone valves and L valves for controllingand driving the flow of catalytic particles 48 into and out of thereactor 10. Therefore, no further explanation of such devices andmechanisms need be required herein. The flow rate of the catalyticparticles 48 is controlled independently of gas flow in the reactor 10,and flow rates of each gas 50 a-50 g, in one embodiment, may not becontrolled independently of one another, or in an alternate embodimentmay be controlled independently, thereby enabling the process conditionsand parameters to be adjusted on an individual basis.

The present invention contemplates that the reactor 10, as shown anddescribed herein, is constructed so as to enable the gases supplied tothe reactor 10, such as gases 50 a-50 g, to be recycled after havingbeen output from the reactor 10. For example, inert preheating gas 50 ais collected from gas outlet 44 a, purified if necessary, mixed withadditional inert preheating gas 50 a to replace lost gas, reheated andpressurized, and reintroduced at preheating gas inlet 22 a. Similarly,heated reducing gas 50 b is collected from gas outlet 44 b, purified ifnecessary, mixed with additional heated reducing gas 50 b, reheated andpressurized, and reintroduced at reducing gas inlet 22 b. In a similarmanner, reheating gas 50 c is collected from reheating gas outlet 44 c,purified if necessary, mixed with additional reheating gas 50 c,reheated and pressurized and reintroduced at reheating gas inlet 22 c.Further, heated carbon-containing reaction gas 50 d is collected fromgas outlet 44 d, purified if necessary, mixed with additional heatedcarbon-containing reaction gas, reheated and pressurized andreintroduced at reaction gas inlet 22 d. Similarly, heated post reactiongas 50 e is collected from the post reaction gas outlet 44 e, purifiedif necessary, mixed with additional heated post reaction gas 50 e,reheated and pressurized and reintroduced at post reaction gas inlet 22e. Cooling gas 50 f is collected from cooling gas outlet 44 f, purifiedif necessary, mixed with additional cooling gas 50 f, cooled,pressurized and reintroduced at cooling gas inlet 22 f. Finally, heatedcombustion gas 50 g is collected from combustion gas outlet 44 g,purified to remove combustion products such as CO₂, mixed withadditional heated combustion gas 50 g, reheated and pressurized, andreintroduced at combustion gas inlet 22 g. Methods of mixing gases,purifying them, and reheating and repressurizing them are known topersons of ordinary skill in the art, so further discussion herein ofsuch methods is not deemed necessary.

As noted herein, the apparatus shown in FIGS. 2 and 3 and in the portionof the present specification relating thereto describes but one type ofapparatus which may be employed to carry out the process contemplatedherein. Other apparatuses which may also be used are shown in FIGS. 4and 5 and are further described below.

FIG. 4 shows an apparatus 58 comprising a reactor 60 used as a componentin a continuous fluidized bed process. Catalytic particles 82 are fedvia an input conduit 62 into a reducing chamber 64 and are reduced in amanner similar to that discussed previously. A reducing gas as discussedpreviously (such as H₂) can be input through a gas inlet 68 and removedthrough a gas outlet 70. After the reduction step, the catalyticparticles 82 can be fed, via any appropriate mechanism, through anoutput channel 66 into a reheating chamber 72 wherein the catalyticparticles 82 are heated to an appropriate reaction temperature via aninert heating gas as discussed previously (such as He) which isintroduced into the reheating chamber 72 via a gas inlet 76 and whichcan be removed via a gas outlet 78. After heating, the catalyticparticles 82 are passed via an output channel 74 into the reactor 60wherein they are subjected to reaction conditions by inputting acarbon-contained gas as discussed previously (such as CO) at a highspace velocity via a gas inlet 80, which results in the catalyticparticles 82 being maintained as a “fluidized bed” 83 in which thecarbon nanotube formation process occurs. Light catalytic particles 85may be lofted out of the fluidized bed 83 and carried out with anexhaust gas through an exhaust conduit 84 into a light particle trap 88,which filters the light catalytic particles 85 from the exhaust gas,which is eliminated via an exhaust outlet 90. The light catalyticparticles 85 are thereby recovered and passed through a trap output 92via a light particle conduit 94 into a catalytic particle treatment unit96 for further processing and recycling of the light catalytic particles85. Meanwhile, the catalytic particles 82 which comprise the fluidizedbed 83, after an appropriate exposure to reaction conditions within thereactor 60, are removed from the reactor 60 via a particle output 86 andenter a cooling chamber 98 wherein an inert cooling gas as discussedpreviously (such as He) provided at a lower temperature is introducedvia a gas inlet 102, thereby cooling the reacted catalytic particles 82.The cooling gas is removed via a gas outlet 104. The catalytic particles82 then leave the cooling chamber 98 via an output conduit 100 and enteran oxidation chamber 105. In the oxidation chamber 105, the catalyticparticles 82 are exposed to an oxidative gas as described herein before(such as O₂) which enters via a gas inlet 106, the amorphous carbonresidue on the catalytic particles 82 being removed in the oxidationchamber 105. Gases are eliminated from the oxidation chamber 105 via agas outlet 107, and the catalytic particles 82 leave via an outputconduit 108 and pass through a particle conduit 110 into the catalyticparticle treatment unit 96. In the catalytic particle treatment unit 96,the catalyst is separated from the support component of the catalyticparticles 82 and 85, and the carbon nanotubes are separated from thecatalyst by processes previously discussed. The carbon nanotubes areoutput via a product output 112 for additional purification. Thecatalyst and support components are transferred via a separation outputconduit 114 to a catalyst and a support recovery unit 116 wherein thecatalyst is recovered, such as by precipitation, for example, and thesupport is recovered, such as by precipitation, for example, and thecatalyst and support are reconstituted in a manner previously describedto form catalytic particles 82 which can be reused in the process. Thecatalytic particles 82 thus recovered are transferred via a feedingconduit 118 back into the reducing chamber 64 for reuse, and may bemixed with fresh catalytic particles 82 which enter via a freshcatalytic particle input 120. As previously explained, the gases used inthe apparatus 58 of FIG. 4 are preferably recovered and recycled for usewithin the apparatus 58.

FIG. 5 shows an apparatus 128 which comprises a reactor 130 used as acomponent in a quasi-continuous batch and fluidized bed process.Portions of the apparatus 128 rely on batch-type processes whileportions rely on a fluidized bed-type process, as explained below.Catalytic particles 144 are fed via an input conduit 132 into areducing/heating chamber 134 wherein the catalytic particles 144 arereduced in a manner similar to that discussed previously except that abatch process is utilized rather than a continuous process. Thecatalytic particles 144, having been reduced, are then reheated in thesame reducing/heating chamber 134 in which they were reduced. The gasesused for reducing and heating are introduced via a gas inlet 138 and areremoved via a gas outlet 140. The reducing process thereby alternateswith the reheating process. After reheating, the catalytic particles 144pass out of the reducing/heating chamber 134 via output conduit 136 andpass through a reactor input 142 into the reactor 130 where they areexposed to a carbon-containing gas at a high space velocity via a gasinlet 149, thereby forming the catalytic particles 144 into a fluidizedbed 150 as described previously for the apparatus 58 of FIG. 4, thecarbon nanotube formation process beginning in the fluidized bed 150. Aswith the fluidized bed process described above, light catalyticparticles 145 may be lofted out of the fluidized bed 150 and carried outwith exhaust gas through an exhaust conduit 146 into a light particletrap 151 which filters the light catalytic particles 145 from theexhaust gas, which is eliminated via an exhaust outlet 152. The lightcatalytic particles 145 are thereby recovered and passed through a trapoutput 154 via a light particle conduit 156 into a catalytic particletreatment unit 158 for further processing and recycling of the lightcatalytic particles 145. Meanwhile, the catalytic particles 144 whichcomprise the fluidized bed 150 are removed from the reactor 130 via aparticle output 148 after an appropriate exposure to reaction conditionswithin the reactor 130 and enter a cooling/oxidizing chamber 160,wherein an inert cooling gas (such as He) provided at a lowertemperature is introduced via a gas inlet 166, thereby cooling thereacted catalytic particles 144. The cooling gas is removed via a gasoutlet 168. The catalytic particles 144, having been cooled, can now beexposed to an oxidative gas (such as O₂) via the gas inlet 166, whereinamorphous carbon residues on the catalytic particles 144 are removed.Gases are eliminated from the cooling/oxidizing chamber 160 via the gasoutlet 168, and the now oxidized catalytic particles 144 leave via anoutput conduit 162 and pass through a particle conduit 164 into thecatalytic particle treatment unit 158. In the catalytic particletreatment unit 158, the catalyst is separated from the support componentof the catalytic particles 144 and 145, and the carbon nanotubes areseparated from the catalyst by processes previously discussed. Thecarbon nanotubes are output via a product output 170 for additionalpurification. The catalyst and support components are transferred via aseparation output conduit 172 to a catalyst and support recovery unit174 wherein the catalyst is recovered, such as by precipitation, and thesupport is recovered, such as by precipitation, and the catalyst andsupport are reconstituted in a manner previously described to formcatalytic particles 144 which can be reused in the process. Thecatalytic particles 144 thus recovered are transferred via a feedingconduit 176 back into the reducing/heating chamber 134 for reuse, andmay be mixed with fresh catalytic particles 144 which enter via a freshcatalytic particle input 178. As previously explained, the gases used inthe apparatus 128 of FIG. 5 are preferably recovered and recycled foruse within the apparatus 128.

Effects of Operating Conditions on the Reaction Yield of SWNTs

EXAMPLE 1 Optimal Reaction Conditions

The SWNTs are obtained through the following exemplary exothermic andreversible reaction known as the Boudouard reaction:2CO(g)⇄C(SWNT)(s)+CO₂(g)

This reaction is exothermic with a ΔH_(R) of −41,220 cal/mol at 298° C.and an equilibrium constant of 0.047 psi⁻¹ at 700° C.

Under the reaction conditions, the Co:Mo catalyst would be expected todeactivate due to different phenomena, such as the formation of theSWNTs themselves, the formation of other carbon species and/or thereduction of the catalyst by the carbon-containing gas.

Since the reaction and the deactivation occur at the same time, in orderto maximize the yield of the reaction, it is important to find theconditions under which the formation of the SWNTs is much faster thanthe deactivation of the catalyst. Many of those conditions aredetermined by the fact that this reaction is exothermic and reversible.

To calculate the equilibrium conversions under different conditions, theequilibrium constants of the Boudouard reaction were determined atdifferent temperatures using the process simulator software PROVISION/IIversion 5.0 (from SIMSCI) with the Soave-Redlich-Kwone property package.The properties (ΔH_(f), ΔGf, cp, etc.) of the Graphite were used as theproperties of the SWNT. The equilibrium conversion x was then estimated,based on the following definition:x=(F° _(CO) −F _(CO))/F° _(CO)  (1)where F_(CO) is the molar flow rate of CO in a continuous reactor whenthe equilibrium is reached and F°_(CO) is the initial molar flow rate ofCO that is fed into the reactor. The equilibrium conversions, obtainedin such a reactor at constant pressure and with pure CO in the feed,were obtained solving the following equilibrium equation andstoichiometric balances:Keq=P _(CO) ₂ /(P _(CO))²  (2)P _(CO) =y _(CO) ·P _(T)=[(1−x)/(α⁻¹−0.5·x)]·P  (3)P _(CO) ₂ =y _(CO) ₂ ·P _(T)=[(0.5·x)/(α⁻¹−0.5·x)]·P  (4)where α is the molar fraction of CO in the feed stream, y_(CO) andy_(CO) ₂ are the molar fractions of CO and CO₂ in the gas phase whenequilibrium is reached, P_(CO) and P_(CO) ₂ are the partial pressures ofCO and CO₂ in equilibrium and P_(T) is the total pressure under whichthe reactor operates. Combining equations (1) to (4), the followingsolution (5) can be obtained:x=(2·K _(eq) ·P+0.5·α⁻¹)−√(2·K _(eq) ·P+0.5·α⁻¹)² −K _(eq) ·P·(4·K _(eq)·P+1)/(2·K _(eq) ·P+0.5).

Using this equation (5), the equilibrium conversion as a function oftemperature and pressure can be readily calculated.

Although high temperatures (above 650° C.) are necessary in order toproduce SWNTs with high selectivity, since the reaction is exothermic,the inverse reaction of the nanotube formation increases if thetemperature is too high (e.g., above about 850° C.) and the overallreaction rate is lower (the equilibrium of the reaction shifts to theleft). For example, K_(eq) is 0.57 psi⁻¹ at 600° C., 0.047 psi⁻¹ at 700°C., and 0.0062 psi⁻¹ at 800° C., and at atmospheric pressure, asillustrated in FIG. 6 the equilibrium conversion of a stream of pure COis 83% at 600° C., but only 15% at 800° C. and only about 1% at 1000° C.

It is important to note that if the inverse reaction is avoided (e.g.,by maintaining a low CO₂ concentration), according to the Arrhenius Law,the higher the temperature, the higher the reaction rate. The upperlimit for the temperature will be determined in this case by thedeactivation of the catalyst due to sintering.

Since the mole number in the gaseous phase is higher in the left term ofthe equation than in the right term, as pressure increases, overallreaction rate of SWNT production increases, and the equilibrium of thereactions shifts to the right as illustrated in FIG. 6. For instance, ifthe reaction is carried out isothermically starting with pure CO at 700°C., the conversion of the CO at the equilibrium shifts from 48% to 75%when the pressure is increased from 14.7 psi to 150 psi. However, athigher temperatures, the conversions are very low, and the effect ofpressure is less pronounced. Therefore, the pressure needed to keep amoderate equilibrium conversion at temperatures above 800° C. is indeedhigh. For example, 300 psi are needed to reach 30% conversion at 900° C.

The CO₂ produced during the reaction also plays a very important role.Since CO₂ is a reaction product, its presence shifts the equilibriumtowards the side of the reactants. The CO₂ not only dilutes the CO (thereaction gas) but it also increases the importance of the inversereaction. Both phenomena contribute to a lower reaction rate and caneven inhibit the reaction completely if equilibrium conditions arereached. As mentioned above, the effects of CO₂ are exacerbated withhigher temperature and lower pressure, as well as lower concentration ofCO in the feed (as discussed in detail herein below). For example, usingequation (5) it can be shown that at 800° C. and 14.7 psi, a COconversion as low as 14.3% (corresponding to a CO₂/CO ratio as low as0.083) is enough to inhibit the reaction, even if there is no other gaspresent. Since CO₂ is produced during the production of SWNTs, it isimportant to use high flow rates (high space velocities) of the reactivegas (CO) in order to maintain a low CO₂/CO ratio during the process.

The presence of an inert gas in the fed stream also may have undesirableeffects. It not only decreases the reaction rate by diluting thereaction gas, but it also shifts the equilibrium of the reaction to theleft, reducing the overall reaction rate even more due to the effect ofthe inverse reaction and lowering the overall conversion. FIG. 7illustrates this trend, showing the conversion in the equilibrium fordifferent CO concentrations in the fed stream when the reaction iscarried out isothermically at 800° C. At atmospheric pressure (14.7psi), for example, the equilibrium conversion shifts from 14.3% whenpure CO is used to 1.7% when the concentration of CO is reduced to 10%.

As stated above, the flow rate of the reaction gas (CO) can affect theyield of the reaction. Two factors are important in determining theoptimal CO flow rate: the external diffusional effects in the catalystparticle and the conversion of CO to CO₂. Since the reaction isexothermic, if the flow rate is not high enough to avoid the externaldiffusional effects, the real temperature inside the particle may behigher than the temperature in the bulk of the gas phase, anddeactivation due to thermal effects (e.g., sintering) may becomeimportant. Generally, the changes in concentration in the external layeraround the catalyst particles are less pronounced than the temperatureprofiles. However, with this reaction, they can also be significant dueto the net molar flow towards the inside of the particle that isgenerated during the reaction by the net reduction in the number ofmoles. This phenomenon would increase the CO₂/CO ratio inside thecatalyst particle, thus limiting the equilibrium conversion. The secondimportant point is the effect of space velocity on the CO conversion. Asmentioned above, in order to reduce the CO₂ concentration, high flowrates are required.

Therefore, especially preferred operating conditions are a high reactivegas concentration, a temperature in the range of from about 650° C. toabout 950° C., high pressure (above about 70 psi), and a high spacevelocity (above about 30,000 h⁻¹).

Experimental Verification of Optimal Reaction Conditions

EXAMPLE 2 Catalytic Materials

A Co—Mo catalyst with a Co:Mo molar ratio of 1:3 was prepared byimpregnating a silica gel support (SiO₂, Aldrich, 70-230 mesh, averagepore size 6 nm, BET area 480 m²/g, pore volume 0.75 cm³/g), with aqueoussolutions of Cobalt Nitrate and Ammonium Heptamolybdate to obtain thebimetallic catalyst. The liquid/solid ratio was kept atincipient-wetness conditions, which for this support corresponds to 0.63cm³/g. The total metal content was 6 wt %. After impregnation, thecatalysts were first dried in air at room temperature, then in an ovenat 120° C., and finally calcined in flowing air at 500° C.

Two batches of catalysts with different particle sizes were preparedusing different grounding techniques at the moment of the impregnation.In the first batch, the original particle size of the impregnatedsupport was used (i.e., particle size about 212-62 microns, 70-230mesh). In the second batch, the particle size was reduced by grindingthe particles into a fine powder (i.e., particle size smaller than 75microns, 200 mesh).

Reaction

The reaction was carried out in three different reactors. The firstreactor (a) consisted of a horizontal quartz tube of 1 inch diameter, inwhich a ceramic boat with 0.5 g of calcined catalyst (powder) wasplaced. The second reactor (B) and the third reactor (C) were typicalquartz fixed-bed reactors of ⅛ and ¼ inch in diameter, respectively. Inreactor B, the amount of catalyst used was 0.05 g of the powder form,while in reactor C, the amount was 0.5 g of the 70 mesh material. In allthree cases, before reaction the catalysts were heated in H₂ up to 500°C. at 8° C./min, maintained under such conditions for 30 minutes andthen further heated in He up to 700° C. or 800° C. Subsequently, CO wasintroduced at the same temperature at varying flow rates. After a givenreaction period that ranged from 1 to 120 minutes, the reactor wasflushed in He and cooled down to room temperature.

EXAMPLE 3 Characterization of Carbon Deposits

A combination of transmission electron microscopy (TEM), scanningelectron microscopy (SEM), Raman Spectroscopy and Temperature ProgrammedOxidation (TPO) techniques was used to characterize the carbon speciesproduced during the reaction period. The TEM images were obtained in aJEOL 2000FX-TEM. For these observations, the specimens were prepared bysonicating the samples in isopropanol for 10 minutes and then depositinga few drops of the resulting suspension on a TEM grid. The laser Ramanspectra were obtained in a JASCO TRS-600SZ-P single monochromatorspectrophotometer, equipped with a CCD (charge-couple device) with thedetector cooled to about 153 K with liquid nitrogen. The excitationsource was the 514.5 nm line of a Spectra 9000 Photometrics Ar ionlaser. The TPO measurements, which also allowed quantification of theamount of the different species of carbon deposited on the catalyst,were conducted by passing a continuous flow (50 cm³/min) of 2% O₂ in Heover the catalyst containing the carbon deposits while the temperaturewas linearly increased at a rate of 12° C./min. The CO₂ and CO producedduring the oxidation was quantitatively converted to methane in amethanator, where the stream coming from the TPO was mixed with a 50cm³/min stream of H₂ over a 15% Ni catalyst supported on γ-Al₂O₃ at 400°C. The evolution of the methane produced in the methanator thatcorresponded to CO₂ and CO generated in the TPO was monitored in a SRImodel 110 FID. Quantification of the CO₂ generated was achieved bycalibration with pulses of pure CO₂ and oxidation of known amounts ofgraphite.

EXAMPLE 4 Effects of Flow Rate and CO Concentration

The experimental results obtained in a series of runs under differentconditions verify that indeed the production of SWNT is restricted bythe equilibrium of the Boudouard reaction. FIG. 8 shows the variation ofcarbon yield as measured by the TPO method as a function of time onstream using different flow rates. These runs were conducted in thefixed-bed reactor B, which was loaded with 0.05 g of the Co:Mo/SiO₂catalyst and either pure CO or 50% CO in He. Also, a run conducted inthe reactor with the ceramic boat inside (reactor A) is included forcomparison.

It can be observed that although the final yield is almost the same forall cases in which the concentration of CO is 50%, the initial reactionrate (given by the slopes of the curves at t=0) is much higher for theruns conducted in the fixed-bed reactor than for the runs done inreactor A. When 100% CO was used, the behavior obtained was somewhatdifferent. Not only the initial reaction rates but also the final yieldswere significantly increased when the flow rate was increased. Thereason for such an increase may be one or more of the following three:a) diffusional effects, which are lower when the flow rate is increased;b) effect of the reverse reaction due to lower concentration of CO₂ andpressure rise in the system due to the pressure drop developed in thefine-powder catalytic bed.

The effects of external diffusional limitations are clearly seen in thelarge differences observed between the yields obtained in reactors A andB. The one with the boat inside (reactor A) is likely to have a lessuniform flow pattern with stagnant zones than the flow-through fixed bedreactor (reactor B). The influence of pressure was independentlyinvestigated by conducting two experiments under the same conditionsexcept for pressure, which was 15 psi for one case and 75 psi for theother. This comparison is made in FIG. 9, which shows that when thepressure increases, both the initial reaction rate and the final yieldincrease.

EXAMPLE 5 Carbon Profiles Along the Reactor

Several different runs were conducted in reactor C, which as mentionedabove was loaded with a larger amount of catalyst with bigger catalystparticles. Therefore, the flow resistance of the catalytic bed was muchlower than when using the powder catalyst, resulting in much lowerpressure drop and, consequently, a constant pressure across thecatalytic bed. The use of a longer catalyst bed also allowed samples tobe taken of products from different parts of the reactor. These datagave important information about the deposition of carbon along thereactor bed at constant pressure, but at increasing CO conversion. Theresulting carbon profiles are illustrated in FIG. 10. When the reactionwas carried out at 700° C. and 85 psi with a flow rate of 850 cm³/min ofpure CO, the yield obtained after two hours of reaction was around 11 wt%, with a selectivity toward SWNT of about 80%. It is interesting tonote that the yield was almost constant along the reactor, although theCO₂ being formed would inhibit the Boudouard reaction. This resultindicates that in this case the total conversion of CO is significantlylower than the equilibrium conversion (about 76% for these conditions),making the effect of the reverse reaction negligible, even during thefirst moments of the reaction when the reaction rate is higher.

By contrast, when the temperature was increased to 800° C., the yield inthe first part of the reactor (where there is no CO₂ present) is veryhigh, around 20 wt %, but it decreases along the reactor down to about11 wt % in the last fraction of the bed. Due to the higher temperature,the rate of the forward reaction, which dominates in the first part ofthe reactor, is high and leads to high carbon yields. However, as the COconversion increases along the bed, it reaches the equilibriumconversion, which is around 43% for these conditions, and limits thedeposition of carbon. As mentioned above, since the production of carbonoccurs only during a short initial time, the catalyst deactivationprevents further accumulation of carbon along the bed.

When the reaction was carried out at 800° C. with 50% CO in He, theyield in the first part of the reactor was only around 11 wt %,significantly lower than the yield obtained with pure CO due to thelower reaction rate caused by the dilution. However, the dilution alsoshifts the equilibrium conversion, which is around 28% under theseconditions. Therefore, the yield along the reactor went down as the COconversion increased and thermodynamic limitations begun to play a role.In this case, the yield near the end of the reactor was only about 5 wt%.

EXAMPLE 6 Effect of CO₂

When the dilution of the feed was done with CO₂ instead of He (50% CO inCO₂), there was almost no deposition of carbon, and the final yield wasconstant and around 0.14 wt %. This observation gives strong support tothe idea of reversible carbon deposition since a CO₂/CO ratio of 1:1would correspond to a CO conversion of 66.6% if a pure CO feed wereused. The resulting CO₂ concentration is higher than that in equilibriumat the reaction conditions, which for a pure CO feed would be about 43%.As a result, thermodynamics predicts that the forward reaction would notoccur and no carbon deposition should be expected. To check that theabsence of carbon formation was not due to other phenomena, such ascatalyst deactivation by CO₂, an experiment was performed in which pureCO₂ was flowed through the reactor for 90 seconds under the reactionconditions (800° C. at 85 psi) before starting the reaction. As shown inFIG. 10, the yield profile along the reactor was practically the same asthe one obtained without the pretreatment in CO₂.

It must be noted that in all cases presented herein for which the carbonyield has been increased up to 20 wt %, the selectivity to single wallnanotubes has been very high, as demonstrated by TPO, TEM, SEM and RamanSpectroscopy.

In conclusion, the present invention, in a preferred version, isdirected to a process for producing single walled carbon nanotubes. Themethod comprises the steps of (1) feeding catalytic particles into areactor, wherein the catalytic particles comprises a support materialand a metallic catalyst, and wherein the metallic catalyst is effectivein catalyzing the conversion of a carbon-containing gas into singlewalled carbon nanotubes, (2) removing air from the catalytic particlesby exposing the catalytic particles to an inert preheating gas underelevated pressure, (3) reducing the catalytic particles by exposing thecatalytic particles to a heated reducing gas under elevated pressure,thereby forming reduced catalytic particles, (4) preheating the reducedcatalytic particles to a reaction temperature by exposing the reducedcatalytic particles to a reheating gas under elevated pressure, (5)catalytically forming carbon nanotubes by exposing the reduced catalyticparticles to a carbon-containing gas heated to the reaction temperatureand under a space velocity of 30,000 h⁻¹ or higher for a duration oftime sufficient to cause catalytic production of single walled carbonnanotubes, thereby forming reacted catalytic particles bearing thesingle walled carbon nanotubes, (6) flushing the carbon-containing gasfrom the reacted catalytic particles by exposing the reacted catalyticparticles to a heated post reaction gas under elevated pressure, (7)cooling the reacted catalytic particles by exposing the reactedcatalytic particles to a moderate temperature cooling gas under elevatedpressure, (8) removing amorphous carbon deposited on the reactedcatalytic particles by exposing the reacted catalytic particles to aheated O₂-containing gas under elevated pressure, thereby oxidizing theamorphous carbon and forming semi-purified catalytic particles, (9)treating the semi-purified catalytic particles to separate the supportmaterial from the catalyst, (10) treating the catalyst with a solvent todissolve the metallic catalyst, thereby separating the single walledcarbon nanotubes from the catalyst, (11) recovering the support materialby precipitation, (12) recovering the catalyst by precipitation, (13)combining the recovered support material and the recovered catalyst,(14) impregnating the catalyst onto the support material to regeneratethe catalytic particles, and (15) feeding the regenerated catalyticparticles into the reactor. The process may be a continuous flowprocess.

The metallic catalyst of the catalytic particles may be a Co—Mo catalysthaving a Co—Mo molar ratio in the range of from about 1:2 to about 1:4.The support material of the catalyst may be SiO₂, Al₂O₃, MgO, ZrO₂,zeolites, MCM-41, or Mg(Al)O, and is preferably SiO₂.

The process may further comprise the step of calcining and pelletizingthe support material before or after the support material is impregnatedwith the catalyst.

Each gas may have a gas flow rate which can be controlled independentlyof a flow rate of the catalytic particles. In the process the elevatedpressure may be defined as a pressure above about 70 psi. The preheatinggas may be He, Ar, N₂, other inert gases and mixtures thereof, and mayhave a temperature in the range of from about 500° C. to about 1200° C.,or more particularly, about 700° C. to about 1000° C., and morepreferably about 750° C. to about 950° C.

The reducing gas may be H₂, NH₃, CH₄ and mixtures thereof, and have atemperature in the range of from about 400° C. to about 800° C., or moreparticularly, about 500° C.

The reheating gas may be He, Ar, N₂, other inert gases, and mixturesthereof, and may have a temperature in the range of from about 600° C.to about 1200° C., and more particularly, about 750° C. to about 950° C.

The carbon-containing gas may be carbon monoxide, methane, ethane,propane, butane, hexane, ethylene, propylene, acetone, methanol,toluene, benzene, napthalene, and mixtures thereof.

Where the carbon-containing gas is carbon monoxide, the concentrationmay be in the range of from about 50% to about 100%.

The reaction temperature may be in a range of from about 700° C. toabout 1000° C., or more particularly, about 750° C. to about 950° C.

Preferably, the carbon-containing gas is provided at a high spacevelocity to minimize CO₂ concentration, and particularly is above about30,000 h⁻¹.

The post reaction gas may be He, Ar, N₂, other inert gases, and mixturesthereof, and may have a temperature of in the range of from about 300°C. to about 900° C., and preferably about 750° C.

The cooling gas may be He, Ar, N₂, other inert gases, and mixturesthereof, and may have a temperature in the range of from about 0° C. toabout 300° C., and preferably about 22° C.

The percentage of O₂ in the O₂-containing gas may be in the range offrom about 2% to about 5%, and may have a temperature in the range offrom about 280° C. to about 320° C., and more particularly, about 300°C.

The process may comprise the step of recycling the carbon-containing gasremoved from the reactor after the catalysis step and reusing thecarbon-containing gas in the catalysis step, recovering the preheatinggas, the reducing gas, the reheating gas, the carbon-containing gas, thepost reaction gas, the cooling gas, and the O₂-containing gas aftertheir exit from the reactor, purifying each of said gases, and reusingeach of said gases in the reactor. And in the step of combining therecovered support material and the recovered catalyst, additionalsupport material and/or catalyst may be added before regenerating thecatalytic particles.

Changes may be made in the construction and the operation of the variouscomponents, elements and assemblies described herein or in the steps orthe sequence of steps of the processes described herein withoutdeparting from the spirit and scope of the invention as defined herein.

1. A process for producing single walled carbon nanotubes, comprising:providing a fluidized bed reactor; disposing catalytic particles intothe fluidized bed reactor wherein the catalytic particles comprise asupport material and a catalyst, the catalyst effective in catalyzingthe conversion of a carbon-containing gas into carbon nanotubes; heatingthe catalytic particles in the fluidized bed reactor to a reactiontemperature; and exposing the catalytic particles to a carbon-containinggas at a flow rate sufficient to maintain the catalytic particles as afluidized bed and maintaining the catalytic particles at a temperaturesufficient to cause catalytic production of single walled carbonnanotubes on the catalytic particles thereby forming reacted catalyticparticles bearing the single walled carbon nanotubes.
 2. The process ofclaim 1 wherein the catalyst comprises a transition metal.
 3. Theprocess of claim 1 wherein the catalyst comprises at least a pair oftransition metals.
 4. The process of claim 1 wherein the catalystcomprises a Group VIII metal and a Group VIb metal.
 5. The process ofclaim 1 wherein the catalyst comprises cobalt and molybdenum.
 6. Theprocess of claim 1 comprising the step of reducing the catalyticparticles by exposing the catalytic particles to a heated reducing gasforming reduced catalytic particles.
 7. The process of claim 1 whereinthe step of heating the catalytic particles comprises exposing thecatalytic particles to a heated inert gas.
 8. The process of claim 1further comprising the step of removing the reacted catalytic particlesfrom the reactor.
 9. The process of claim 1 further comprising the stepof cooling the reacted catalytic particles.
 10. The process of claim 1further comprising the step of removing amorphous carbon from thereacted catalytic particles and treating the reacted catalytic particlesto obtain the single walled carbon nanotubes.
 11. The process of claim 1wherein the reaction temperature is about 700° C. to about 1000° C. 12.The process of claim 1 wherein the reaction temperature is about 750° C.to about 950° C.
 13. The process of claim 1 wherein thecarbon-containing gas comprises carbon monoxide, a saturated aliphatichydrocarbon, an unsaturated hydrocarbon, an oxygenated hydrocarbon, analcohol, and/or an aromatic hydrocarbon.
 14. The process of claim 1wherein the carbon-containing gas further comprises a diluent gas. 15.The process of claim 1 comprising the step of treating the reactedcatalytic particles to obtain the single walled carbon nanotubes. 16.The process of claim 1 wherein the carbon-containing gas comprises a gasselected from the group consisting of CO, CH₄, C₂H₄, C₂H₂, or mixturesthereof.
 17. The process of claim 1 wherein the support material of thecatalytic particle is selected from the group consisting of SiO₂, Al₂O₃,MgO, ZrO₂, a molecular sieve, zeolites, MCM-41, an oxidic support, andaluminum-stabilized magnesium oxide.
 18. The process of claim 1 whereinthe catalyst comprises at least one of the metals selected from thegroup consisting of Co, Mo, Ni, Fe, W, or Nb.
 19. The process of claim 1wherein the catalyst comprises a Group VIII metal selected from thegroup consisting of Co, Ni, Ru, Rh, Pd, Ir, Fe, Pt, and mixturesthereof, and a Group VIb metal selected from the group consisting of Cr,Mo, W, and mixtures thereof or a Group Vb metal selected from the groupconsisting of V, Nb and Ta, and mixtures thereof.
 20. The process ofclaim 1 wherein the carbon-containing gas is exposed to the catalyticparticles at a space velocity above 30,000 h⁻¹.
 21. A single walledcarbon nanotube produced by the process of claim 1.